An epoxy resin has the following characteristics: the mechanical strength, heat resistance, chemical resistance, adhesive property, and electrical insulating property are superior, and cure shrinkage is small. Therefore, the same is applied to various applications, such as a semiconductor sealing material, a laminating material for an electronic substrate, an insulating powder paint, an adhesive, and a carbon fiber composite material.
In an epoxy resin applied to such applications, a curing agent is used for curing, and further a curing accelerator may be occasionally used together to progress smoothly a curing reaction and shorten a processing time. A curing accelerator functions not only to accelerate a processing speed of an epoxy resin, but also to improve various physical properties of a resin, such as hardness and strength. As a curing accelerator, an imidazole compound, such as 2-ethyl-4-methylimidazole, a tertiary amine, such as diazabicycloundecene, and a phosphine, such as triphenylphosphine, are used generally.
With such a curing accelerator, the curing acceleration action develops by heating. However, for example, in the case where an epoxy resin, which is liquid at room temperature, is used, a curing reaction progresses gradually at a transport or storage temperature in a range of from room temperature to approx. 50° C. in the epoxy resin containing a curing agent and a curing accelerator, and the viscosity of the resin composition may increase 10 times as high as the initial viscosity, or even higher within a day or so. Further, in the case where a curing agent and a curing accelerator are melt-blended with a solid epoxy resin, the blend may cure as it is during transport and storage in a range of from room temperature to approx. 50° C., and its molding may become difficult.
As described above, the epoxy resin composition containing a curing accelerator has been inferior in terms of handling property and also storage stability.
To cope with the above, as a curing accelerator with improved storage stability, a microcapsuled type, in which the imidazole compound or the like is used as a core (for example, see PTL 1 and 2), a type, in which the tertiary amine is made less soluble in a form of a salt, or an imidazole compound having introduced a substituent for raising a melting point, such as 2-phenyl-4,5-di(hydroxymethyl)imidazole and 2-phenyl-4-methyl-5-(hydroxymethyl)imidazole, is utilized (for example, see PTL 3). The above are so-called high latency type curing accelerators, in which the reactivity is suppressed and its curing acceleration action is exhibited only under a predetermined condition.